1. Field of the Invention
The present invention relates to a method for controlling a screw injecting apparatus such that vibrations are given to a molding material to thereby decrease its viscosity, as well as to such a screw injecting apparatus.
2. Description of the Related Art
A typical prior screw injecting apparatus is shown in section in FIG. 9 hereof. The screw injecting apparatus 100 comprises a heating cylinder 101, a screw 102 received in the heating cylinder 101 rotatably and movably back and forth, an injecting cylinder 103 for moving the screw 102 back and forth, and a hydraulic motor 105 for rotating the screw 102 by means of a piston rod 104 of the injecting cylinder 103. In the injecting apparatus thus arranged, plasticizing-metering phase, waiting phase and injecting phase as described are carried out:
Plasticizing-Metering Phase: Raw molding materials are fed from a hopper 107 into the heating cylinder 101 during. rotation of the screw 102 and heated by the heating cylinder 101 while being transferred toward a discharging nozzle 108 by rotation of the screw 102. By friction heat arising from the transfer and heat transmitted from the heating cylinder 101, the molding materials are plasticized and kneaded. The screw 102 is pushed back rightwardly by a counter force of a molten material accumulated around the tip of the nozzle 108. The amount of the molten material is measured by metering the retreating stroke of the screw 102.
Waiting Phase: After completion of the metering, the hydraulic motor 105 and the screw 102 are held inactive until the molten molding material becomes ripe for injection.
Injecting Phase: By the action of the injecting cylinder 103, the screw 102 is advanced at one stroke to cause the molten molding material accumulated forwardly of the screw 102 to be injected through the nozzle 108 into a mold not shown.
It is important to fill up cavities of the mold before solidification of the molten molding material therein progresses. Thus, the faster the injection speed becomes, the better. To speed up the injection, one may propose (1) to make the injection cylinder more pressurized, (2) to make the injection cylinder have an increased diameter, (3) to melt the molding material at an increased temperature, or (4) to decrease the viscosity of the molten molding material without relying on temperature increase.
The proposals (1) and (2) lead to up-sizing of the apparatus, thereby increasing the cost of production.
In the case of the proposal (3), the viscosity decreases by increase of the melting temperature. However, as can be seen from p-v-T (pressure-volume-temperature interrelation) characteristics, the density decreases as the temperature increases. Thus, volume variation becomes large when the temperture is changed from a molding temperatue to a normal temperature. To compensate for the large volume variation, a large pressure becomes necessary. In a normal injection molding process, pressure control is effected through a gate, whereby the pressure distribution of the resin within the cavities of the mold becomes non-uniform. This non-uniformity becomes more significant when the pressure is increased. Thus, the proposal to increase the melting temperature of the molding material. Rather, the temperature of the molding material should be as low as possible so that the temperature difference between the molding material and the mold can be kept to a minimum.
Consequently, the proposal (4), that is, to decrease the viscosity of the molten molding material without relying on temperature increase, has been seriously noted by researchers. The present inventors have continuously researched to realize the proposal (4) by means of mechanical measures.
FIG. 10 hereof graphically shows the results of such inventors' research. As can be readily appreciated from the figure, the inventors have found during the research that the viscosity of the resin material significantly changes when the material is vibrated by a vibration frequency in a given range.
More specifically, vibration frequencies are shown in herz (Hz) along the horizontal axis while the material viscosity is shown in poise along the vertical axis. PMMA (poly(methyl methacrylate)), a typical resin material, was imparted various vibrations while maintaining it at 240.degree. C. While it exhibits the viscosity of 126.times.10.sup.3 poise at the frequency of 0 Hz, the PMMA exhibited the viscosity of 65.times.10.sup.3 poise at 5 Hz, 14.times.10.sup.3 at 30 Hz, and 9.times.10.sup.3 poise at 55 Hz.
While it exhibits the viscosity of 63.times.10.sup.3 at the frequency of 0 Hz, PC (polycarbonate) exhibited 26.times.10.sup.3 poise at 15 Hz, and 15.times.10.sup.3 poise at 40 Hz.
The research has thus revealed that it becomes possible to satisfactorily decrease the viscosity of the resin material by vibrating the material at the frequency ranging from 5 Hz to 40 Hz, desirably at 15 Hz or more.
A technique for applying vibrations to a molding material during injection thereof is known from, for example, Japanese Utility Model Laid-Open Publication No. SHO-63-197113 entitled "SHAPING INJECTION MOLDING APPARATUS WITH SHAKING CAPABILITY". The known apparatus includes an ultrasonic oscillator mounted in a heating cylinder thereof and an ultrasonic wave generator disposed remotely from the heating cylinder for ultrasonically vibrating the ultrasonic oscillator. In response to a high frequency signal, the ultrasonic wave generator generates a ultrasonic wave for vibrating the ultrasonic oscillator. The resulted ultrasonic vibrations are applied to a molten resin material being injected, thereby fully filling up finely-patterned portions and complex-shaped portions of a molded product.
Ultrasonic waves have a frequency above about 20 kHz which is too high for humans to hear. The ultrasonic wave generator is designed to produce such waves. This and the above-described arrangement bring about the following problems:
(a) it is likely that fine bubbles be formed in the molten material, because an extremely high vibration frequency is applied to the molten material, thereby suddenly changing the pressure of the molten material;
(b) there remains a fear that the effect of the vibrations or shaking may not reach deepest portions of the cavities of the mold, because the injecting phase is finished too quickly; and
(c) being disposed within the heating cylinder, the ultrasonic oscillator will present a bar to the flow of the molten material.